Well it's one way to end that story for good. Sometimes the fans or the publishers just won't let go forcing you to plough out more rubbish to sate their demands. Killing off a main character at least allows the studio to move onto somebody else.

Do we really want to see Sheppard fighting space-pirates after s/he just saved (hopefully) the universe from annihilation? After afterwards would seem trivial.

Bioware did say that Mass Effect will continue but it's the end of Sheppard and what's a better way of ending it?